This invention relates generally to additive manufacturing, and more particularly to apparatus and methods for gas flow monitoring and process control in additive manufacturing.
Additive manufacturing is a process in which material is built up layer-by-layer to form a component. Additive manufacturing is limited primarily by the position resolution of the machine and not limited by requirements for providing draft angles, avoiding overhangs, etc. as required by casting. Additive manufacturing is also referred to by terms such as “layered manufacturing,” “reverse machining,” “direct metal laser melting” (DMLM), and “3-D printing”. Such terms are treated as synonyms for purposes of the present invention.
One type of additive manufacturing machine is referred to as a “powder bed” machine and includes a build chamber that encloses a mass of powder which is selectively fused by a laser to form a workpiece. The build chamber is enclosed in a housing that typically includes provisions for a flow of shielding gas therein. The shielding gas is used to transfer heat away from the surface of the power bed, to prevent vaporized powder from condensing on the surface of the workpiece, and to control undesired chemical reaction such as oxidation.
One problem with prior art additive manufacturing machines is that the gas flow rate can be too high or too low. If the gas flow velocity is too high, it can disturb the powder in the build chamber, disrupting the build process. If the gas flow velocity is too low, it will provide insufficient heat transfer and vapor removal, resulting in measurably worse surface roughness in the finished workpiece.
Another problem with prior art additive manufacturing machines is that the gas flow velocity varies over the surface of the build chamber. Specifically, the gas flow decelerates as it passes over the surface, because of normal pressure and friction losses. The velocity may also be inconsistent in a direction perpendicular to flow. Because of this variation, the gas flow rate may be acceptable in one location but too high or low in another.